The present invention relates to the subsurface disposal of sewage and waste water, in particular, to disposal of sewage by means of septic tank type systems and associated leach fields.
Subsurface sewage disposal systems, commonly called septic tank systems or septic systems, are widely used for on-site processing of sewage from dwellings and other smaller volume sewage sources. Typically, sewage is delivered via a sewer line to a septic tank for primary processing. The septic tank effluent, or wastewater, is flowed to a leaching system for secondary processing by means of distribution pipes. The leaching system, also commonly called a disposal field, leach field, or infiltration field, typically comprises permeable soil of the earth and some sort of excavation in the soil which is filled with stone particulate such as crushed stone or coarse gravel (typically 2.5 cm in dimension) and or a mechanical component, the function of which is to convey wastewater through a conduit, to infiltrate it into the soil.
The principal function of the septic tank is to effect primary sewage processing by engendering physical separation and retention of solids which are lighter and heavier than water, typically by settling and baffling. Solids which settle out as sludge are mostly decomposed by action of bacteria in a typically anaerobic environment. Gases which are generated in the process are vented to atmosphere. The wastewater from the septic tank is typically conveyed to the leach field by passing it through a distribution box and piping which channels wastewater to the leach field trenches, in a predetermined fashion. The wastewater is supposed to be free of solids of significant size. It will contain suspended solids of fine size, micro-organisms such as bacterium and viruses, and various chemical constituents.
The purpose of the leach field is generally to cause the wastewater to be treated or renovated, so it can be benignly returned to the hydrologic cycle which characterizes the movement of water into, through, and from soil beneath the surface of the earth. What follows is a simplified version of certain conventional ways of looking at leach field operation phenomena, to provide a conceptual framework for appreciating the invention. It is not intended to be comprehensive nor limiting.
As the wastewater travels from within a leach trench and through the soil in a properly functioning system, it is subjected to natural chemical and biological processes within a xe2x80x9czone of influencexe2x80x9d, which may extend 30-120 cm from the trench interface with the soil. A traditional leach field is comprised of a trench filled with small (2-3 cm) stone pieces. A perforated pipe runs through the stone, delivering the wastewater along the trench. A popular modern type of leach field comprises a series of interconnected arch shaped molded plastic chambers having perforated walls, such as leaching chambers sold under the Infiltrator brand name. See U.S. Pat. No. 5,401,116 of J. Nichols, and U.S. Pat. No. 5,511,903 of J. Nichols et al. Typically, Infiltrator(copyright) chambers are directly buried in a trench in substitution of the stone-and-pipe leaching device.
The leach field must have sufficient capacity to receive and properly process the anticipated flow of wastewater. The steady state capacity, or the infiltration rate, of a leach field is a function of the resistance to wastewater flow of the surfaces of the trench and the surrounding soil, as such may be influenced by hydraulic phenomena other than permeability, such as capillary action. For illustration here, only the sidewall of the trench will be now discussed. If distilled water is processed in sterile soil of a leach field, the infiltration rate is purely a function of the mechanics and hydraulics of the soil. However, in that wastewater contains organic substances, over time, an active, stable, moist biological crust layer frequently grows on surfaces. Of particular interest is the crust layer which occurs on a trench sidewall and within the nearby soil, especially when the layer tends to block openings in leaching system conduits.
The crust, also commonly called a biomat or biocrust, is an organic layer, typically 0.5-3 cm thick. It is normally less permeable than the surrounding soil. Thus, the biomat often significantly determines the long term steady state infiltration capacity of a leach field. The biomat also serves as a filter for bacteria and some suspended solids. In a properly functioning system, the surrounding soil remains desirably unsaturated and aerobic, thus enabling antibiotic attack of any pathogenic bacteria, and more importantly, chemical reactions involving free oxygen. Biomat is thought to aid in filtering things which enter the influence zone, Nitrogen, discharged in human waste, is characteristically passed through any biomat, predominantly as ammonium (NH4+), to be nitrified, or converted to nitrate (NO3) form, in the aerobic environment of the influence zone and adjacent soil. Foreign constituents in the waste water may also sorb and or react with soil constituents; or they may ultimately be only diluted upon return to the ground water. As the waste water is renovated in the influence zone, it moves mostly outwardly and downwardly toward the ambient water table in the earth, Some water may move upwardly into the vadose above the trench by capillarity, evaporative-uptake and plant-uptake. It is usually required that the bottom of the leach field trench be a particular distance above the ambient water table, because sub-optimal sewage treatment conditions exist in the extremely moist soil, the capillary fringe, just above the water table.
In a properly designed, used and maintained septic tank disposal system, once biochemical equilibrium is reached, the capacity of the Icach field remains stable insofar as infiltration or leaching capacity. However, too frequently, a septic tank system will demonstrate insufficient infiltration capacity. Typically, a failure is manifested by escape of wastewater to the surface of the soil, or by a substantial backing up of sewage in the sewer line, One cause of failure can be gross flow of solids from the septic tank into the leach field piping or chamber system, and blockage of the perforations in such components. The typical best remedy for such is to replace or extend the lcach field. Failure can also be manifested by an inability of a given system to handle normal peak loads of sewage which were handled in the past; and by inadequate purification of the wastewater in the influence zone, resulting in pollution of the groundwater. And, even if a system has not failed, it is desirable to guard against failure by having the greatest economically feasible margin of safety against failure,
Among the known causes of some Nilures are the following. The design of the system has become inadequate for the current conditions, either due to growth of a very heavy biomat, a changed character of wastewater, or changed conditions within the soil in the influence zone. For instance, the biological oxygen demand (BOD) of the waste water may have been increased, or the ambient soil conditions changed, so that the desired biochemical conditions for stable aerobic function in the influence zone are no longer obtained. An accumulation of unreacted wastewater within the influence zone limits oxygen transport. Thus, a cascading type of failure mode may ensue, wherein the influence zone gets bigger and bigger as it gets less and less effective.
Thus, there is a need for alternatives to the costly or sometimes physically impossible remedy of adding to or replacing the leaching system. And, if good technology is at hand, the possibility arises for putting in a smaller system initially and reducing cost, for providing greater margin of safety in any given system, or for allowing growth in use of an existing system.
Various approaches to enhance the capacity of leaching systems have been tried, reflecting different concepts of both failure and remedy. Chemical remedies in the forms of solvents, enzymes, and other proprietary formulations, for deposit into the sewer line with sewage, are commercially sold, but most are disdained or ignored by professionals. U.S. Pat. No. 5,588,777 of Laak discloses the injection of soap into the leach field. U.S. Pat. No. 5,597,264 of Laak discloses a method of periodically back flushing the leach field with water. U.S. Pat. No. 4,333,831 of Petzinger describes the type of problem mentioned above, solving it by using evaporation chambers in substitution of any leach field. U.S. Pat. No. 3,907,679 of Yost describes a system in which low pressure air is forced through a septic tank and then into a long coil of waste water piping, so waste water evaporates into the air and is discharged to atmosphere. U.S. Pat. No. 3,698,194 of Flynn describes how air is blown into a conduit of a leach field and vented from risers at the remote end of conduit, to cause evaporation of liquid in, and to dry out grease in, the conduit, during periods when the conduit is not being used for sewage treatment. U.S. Pat. No. 4,013,559 of Johnson describes how air is introduced into the septic tank, flowed through unique vertical concrete panel leaching system units, and then discharged to atmosphere, to encourage aerobic conditions in waste water within the panels. However, none of these prior art technologies seem to have found wide spread use. Thus, there is a continuing need for new ways to enhance the design and performance of leaching fields, both as they are originally installed and for when there are in need of rejuvenating.
An object of the invention is to provide means for improving the function of septic tank type disposal systems and leach fields, to remedy failures, or forestall failure, or improve performance, in ways which are economical and practical. A further object of the invention is to effect desirable biochemical and physical conditions within the influence zone of a leach field. A still further object is to provide a way of sustaining or rejuvenating leach field performance while at the same time enabling continuous use of a septic tank type sewage system.
In accord with the invention, when wastewater is flowed from a primary sewage processing unit, such as a septic lank, through a conduit, and into an influence zone in the soil, as, comprised of air or other biochemically active gas, flows between the conduit and the influence zone, thereby making an effective physical and or chemical change in the zone. In furl accord, the flow of active gas is sufficient in amount to make the composition of gas within the influence zone effectively different from the composition which exists therewithin, in the absence of such flowing. Thus, if the leach field is functioning properly, the invention maintains or improves such; and, if the field is failing, the invention will restore part or all of the function, In one embodiment, air flows from a conduit, into and through the influence zone, in the same direction as the waste water flows. In another embodiment, air flows from the influence zone and into the conduit In both embodiments, a pressure differential is established by an air mover such as a blower or vacuum pump.
In a preferred embodiment, a conduit is pressurized relative to atmosphere, and air flows through the influence zone, the adjacent soil, and ultimately back to atmosphere. If the influence zone is saturated, the pressure of air causes the water in the influence zone to move away from the conduit and the zone is de-saturated. In another embodiment, air flows in the same manner, but as a result of a sub-atmospheric pressure (vacuum) which is created within an auxiliary pipe buried in the soil adjacent to or beneath the trench in which the conduit runs. In preferred practice, for a sewage system embodying typical conventional soils, the differential air pressure between the conduit and atmosphere is at least 2.5 mm water column, to produce a biochemically significant flow into the influence zone. In further accord with the preferred process of the invention, the influence zone is substantially anaerobic in character, and flowing of air or active gas causes the change so that predominantly aerobic. In still further accord with the invention process, the quantity of air or other gas which is flowed into the influence zone provides oxygen is substantially in excess of the stochiometric quantity which is required for oxidation of the oxidizable constituents in the waste water, as such constituents arc typically determined measurement of Oxygen Demand, in particular Biological Oxygen Demand (BOD). Optionally, a gas or liquid substance is added to the air to enhance biochemical activity.
In another embodiment of the invention, air flows from a pressurized auxiliary pipe buried in the soil adjacent the trench, and into a conduit vented to atmosphere. Alternately, the auxiliary pipe is vented to atmosphere and the conduit is connected to a vacuum pump. When there is a buried auxiliary pipe, a membrane is optionally placed within or on the soil, to control the direction in which air travels from or to the soil surface. The air flow of the invention may be maintained continuously or intermittently, with and without simultaneous flow of waste water. in a preferred practice of the invention. To ensure good functioning of a system, a low volume of air is continuously flowed into the sewage system and the air moves through the influence zone contemporaneously with waste water.
In further accord with the invention, apparatus for treating sewage is comprised of a primary unit, such as a septic tank or other kind of reactor, for primary processing of the sewage; a leach field, for receiving waste water effluent of the primary unit, where the leach field is comprised of a trench in the soil, a conduit within the trench, and soil adjacent the trench comprising an influence zone; and a means, such as a blower or vacuum pump, for producing a pressure differential between the conduit and the adjacent soil, where the pressure differential is sufficient to either force water from the soil, or to create biochemically significant gas flow into the influence zone.
In preferred apparatus embodiments, a valve, more preferably a check valve, is placed in a pipe line of the sewage system, to confine the effect of pressure or vacuum to localized parts of the sewage system. In one instance, there is a check valve in the sewer line downstream of the stack vent and upstream of the point at which pressurized air is injected. In another instance, there is a check valve in the pipe running to the distribution box or a distribution pipe and air pressure is injected into the conduits. In still other embodiments, pressure or vacuum is applied to the sewer line running into the septic tank and a check or other valve is present upstream of the point of connection to the sewer line of the source of differential pressure, such as a blower or vacuum pump. The blower is optionally in the stack vent of the system. When the differential pressure required to achieve the desired effect on the influence zone is sufficiently low to make it practical, the check valve which is used is a water-filled portion of the sewer line, such as U-trap or J-trap. Use of a check valve enables use of the system for processing sewage simultaneously with air being flowed into the leach field. A check valve bypass line, temporary storage reservoir and pump are optionally in the sewer line. In another embodiment, air is introduced into the bottom of the leaching trench by a pipe diffuser or by pipes which run lengthwise within the trench. The duration or periods during which air is flowed is optionally controlled by a control system which senses the composition or pressure in vicinity of the influence zone or elsewhere in the sewage system.
The invention is effective in improving the operation of leach fields in a cost-effective way. Leach field performance and biochemistry are improved and maintained through use of the system. The invention can be applied to existing installations and new installations.
The foregoing and other objects, features and advantages of the present invention will become more apparent from the following description of best mode embodiments and accompanying drawings.